# Choosing A Hash Function > Note: this document is still very much a work-in-progress. Currently missing: > - recommendations for hashed containers > - recommendations for a better persistent hash > - recommendations for a secure hash If a hash function with unchanging output is needed, please select from one of the unchanging forever options below. ## Non-cryptographic name | input | output | unchanging forever | notes ---------------------------------------------|-----------------------------|------------|--------------------|------- [`Hash()`][hash] | overloaded | `uint32_t` | no | This function is currently being updated to return `size_t`. [`PersistentHash()`][persistenthash] | overloaded | `uint32_t` | yes | Fairly weak but widely used for persisted hashes. [`CityHash64()`][cityhash64] | `base::span` | `uint64_t` | yes (note 1) | Version 1.0.3. Has some known weaknesses. [`CityHash64WithSeed()`][cityhash64withseed] | `base::span` | `uint64_t` | yes (note 1) | Version 1.0.3. Has some known weaknesses. ## Cryptographic **There are no hashes in `//base` that provide cryptographic security.** name | input | output | unchanging forever | notes -------------------------------|---------------|---------------|--------------------|------- [`MD5String()`][md5string] | `std::string` | `std::string` | yes | **INSECURE** [`SHA1HashString`][sha1string] | `std::string` | `std::string` | yes | **INSECURE** ## Deprecated > Note: CRC32, Murmur2, and Murmur3 will be listed here. Note 1: While CityHash is not guaranteed unchanging forever, the version used in Chrome is pinned to version 1.0.3. [hash]: https://cs.chromium.org/chromium/src/base/hash/hash.h?l=26 [persistenthash]: https://cs.chromium.org/chromium/src/base/hash/hash.h?l=36 [cityhash64]: https://cs.chromium.org/chromium/src/base/hash/city_v103.h?l=19 [cityhash64withseed]: https://cs.chromium.org/chromium/src/base/hash/city_v103.h?l=20 [md5string]: https://cs.chromium.org/chromium/src/base/hash/md5.h?l=74 [sha1string]: https://cs.chromium.org/chromium/src/base/hash/sha1.h?l=22